As a production method of a seamless pipe, various method such as a mandrel mill method, a plug mill method, a Eugene Sejerne method, and a Erhardt Push Bench method are known. The production method of the mandrel mill method, which is superior in all aspects such as productivity, dimensional precision and inner and outer surface quality, of these methods is widely employed.
In the production method of a seamless pipe based on the mandrel mill method, as shown in FIG. 1, a billet 1 is heated to a predetermined temperature (generally 1100 to 1300° C.) in a heating furnace 2 and then subjected to piercing and rolling by a piercer 3 to produce a hollow shell 4. This pierced hollow shell 4 is subjected to drawing and rolling in a mandrel mill 5 to produce a material pipe 4′.
In the mandrel mill 5, the pierced hollow shell 4 is subjected to drawing and rolling with a mandrel bar 6 with a lubricant containing carbon such as graphite or the like applied to its surface inserted into the hollow shell 4. Then, the material pipe 4′ is re-heated to a predetermined temperature (generally 850 to 1150° C.) in a reheating furnace 7 and subjected to finish rolling by a swaging rolling mill 8 such as a stretch reducer or a sizer.
Here, when a material of the material pipe 4′ is low carbon steel such as austenitic stainless steel (SUS 304, SUS 316, etc.) or the like, if the material pipe 4′ is subjected to drawing and rolling with the mandrel bar 6 with a lubricant containing carbon applied to its surface inserted into the material pipe 4′ and re-heated, a carburizing phenomenon, in which a carburized layer having a higher carbon content is formed at an inner surface of the material pipe 4′, occurs.
When this carburized layer remains in pipe products, for example, carbon steel pipe products, the carburized layer becomes an anomalous hardened portion, and cutting becomes difficult for this portion. When the pipe product is austenitic stainless steel, corrosion resistance such as intergranular corrosion resistance is deteriorated.
Accordingly, hitherto, various methods for inhibiting the carburizing of the inner surface of the seamless pipe or for promoting the decarburization of the inner surface of the seamless pipe have been proposed.
For example, it is proposed to limit an amount of the graphite adhering to the surface of the mandrel bar to 100 mg/m2 or less when a hollow shell is subjected to drawing and rolling by the mandrel mill (for example, refer to Japanese Unexamined Patent Publication No. 2000-24706).
However, it is unfeasible in a production line where the graphite lubricant is used once to limit an amount of the graphite adhering to as a trace amount as 100 mg/m2 or less as proposed in the above Japanese Unexamined Patent Publication No. 2000-24706. The reason for this is that when the graphite lubricant is used once, it adheres to a mandrel bar carrying facilities or the like and is suspended in the atmosphere of a plant. It takes immeasurable costs in order to realize the proposed method, so this method is not effective.
Further, a method, in which the lubricant or the carburized layer remaining on the inner surface of the material pipe rolled in the mandrel mill is removed by use of an abrasive or high-pressure water, is proposed (for example, Japanese Unexamined Patent Publication No. 4-111907, Japanese Unexamined Patent Publication No. 6-182427, Japanese Unexamined Patent Publication No. 8-224611, and Japanese Unexamined Patent Publication No. 2001-105007).
However, the method of removing the carburized layer or the like by use of an abrasive is unfeasible since the cost of abrasive such as hone is expensive and it takes the time to grind the material pipe. Further, by the method of removing the lubricant or the like by use of high-pressure water, the material pipe is nonuniformly cooled. And, this material pipe may be bent by reheating and may inhibit an operation.
Furthermore, a method, in which the carburizing is inhibited or the decarburization is promoted by feeding an oxidizing gas into the material pipe in the reheating furnace, is also proposed (for example, refer to Japanese Unexamined Patent Publication No. 8-57505 and Japanese Unexamined Patent Publication No. 8-90043).
However, a temperature of the material pipe at the time when the oxidizing gas is fed or a required flow rate of the oxidizing gas is not disclosed in any Patent Publication described above. In these Patent Documents, it is just disclosed that the oxidizing gas is indefinitely fed into the material pipe in the reheating furnace, and thereby carbon is oxidized to inhibit carburizing or to promote decarburization. As described later, according to the intensive investigations made by the present inventors, it became apparent that the oxidizing gas may have to be fed excessively in order to prevent the carburized layer from being produced depending on a temperature of the material pipe at the time when the oxidizing gas is fed. When an amount of the oxidizing gas to be fed is increased, the unit requirement of the oxidizing gas is increased, which results in an increase in the production cost of the seamless pipe. Further, when a feed rate of the oxidizing gas is increased, since the temperature of atmosphere in the reheating furnace tends to be lowered, large scale combustion facilities become necessary. That is, there is a problem that this method causes an increase in the production cost or the facilities cost.